A fault detection method of motor inter-turn short circuit based on Hidden Markov Model ( HMM) is introduced in this paper, and an order selection strategy for Hidden Markov Model based on information entropy is proposed in the method. The optimal order of Hidden Markov Model is decided according to the relationship between the entropy rates of symbol occurrence probabilities and the order of Hidden Markov Model. A more accurate and more reasonable Hidden Markov Model is obtained with the selected optimal order; at the same time, the model calculation cost is minimized. The method proposed in this paper was applied to the fault detection experiment of motor inter-turn short circuit; and the experiment results indicate that under the condition of ensuring the fault detection accuracy of motor inter-turn short circuit, the model calculation cost can be decreased effectively by reasonably selecting the order of Hidden Markov Model.